The Geography Of Life

When on board H.M.S. "Beagle" as naturalist, I was much struck with certain facts in the distribution of the inhabitants of South America, and in the geological relations of the present to the past inhabitants of that continent. These facts seemed to me to throw some light on the origin of species—that mystery of mysteries, as it has been called by one of our greatest philosophers.

—Charles Darwin, On the Origin of Species

Some of the loneliest places on Earth are the isolated volcanic islands of the southern oceans. On one of them—St. Helena, halfway between Africa and South America—Napoleon whiled away his last five years in British captivity, exiled from his native France. But the islands most famous for their isolation are those of the Juan Fernández archipelago: four small specks of land totaling about 40 square miles and lying 400 miles west of Chile. For it was on one of these that Alexander Selkirk, the real-life Robinson Crusoe, lived out his solitary tenure as a castaway.

Born Alexander Selcraig in 1676, Selkirk was a hot-tempered Scot who took to sea in 1703 as sailing master of the Cinque Ports, a British privateer deputized to plunder Spanish and Portuguese ships.

Worried about the recklessness of his 21-year-old captain and the shoddy condition of the ship, Selkirk demanded to be put ashore, hoping for timely rescue, when the Cinque Ports stopped for food and water at Más a Tierra island in the Juan Fernández group. The captain obliged, and Selkirk was voluntarily marooned, taking ashore only clothes, bedding, some tools, a flintlock, tobacco, a kettle, and a Bible. Thus began four and a half years of solitude.

Juan Fernández was uninhabited, and besides Selkirk the only mammals were goats, rats, and cats, all of them introduced by earlier sailors. But after an initial period of loneliness and depression, Selkirk adapted to his circumstances, hunting goats and shellfish, eating fruits and vegetables planted by his predecessors, making fire by rubbing sticks together, fashioning goatskin clothes, and warding off rats by taming kittens to share his quarters.

Selkirk was finally rescued in 1709 by a British ship, piloted, oddly enough, by the skipper of the original Cinque Ports. The crew was startled by the wild man in goatskins, who had been alone so long that his English could barely be understood. After helping replenish the ship with fruit and goat meat, Selkirk went aboard and made his way back to England. There he teamed up with a writer to produce a popular account of his adventures, The Englishman, said to have inspired Daniel Defoe's Robinson Crusoe.21 Yet Selkirk could not adapt to a sedentary life ashore. He returned to sea in 1720, and died from fever a year later off the African coast.

The contingencies of time and character produced the story of Selkirk. But contingency is also the lesson of a greater story: the story of the nonhuman inhabitants of Juan Fernández and other islands like it. For although Selkirk did not know it, Más a Tierra (now called Alejandro Selkirk Island) was inhabited by descendants of earlier castaways—the Robinson Crusoes of plants, birds, and insects who found their way to the island by accident thousands of years before Selkirk. Unknowingly, he was living in a laboratory of evolutionary change.

Today the four islands of Juan Fernández are a living museum of rare and exotic plants and animals, with many species that are endemic—found nowhere else in the world. Among them are five species of birds (including a giant, 5-inch rust-brown hummingbird, the spectacular and endangered Juan Fernández firecrown), 126 species of plants (including many bizarre members of the sunflower family), a fur seal, and a handful of insects. No comparable area anywhere in the world has so many endemic species. But the island is just as notable for what it is missing: it harbors not a single native species of amphibian, reptile, or mammal—groups that are common on continents throughout the world. This pattern of bizarre and efflorescent forms of endemic life, with many major groups strikingly absent, is repeated over and over again on oceanic islands. And, as we'll see, the pattern gives striking evidence for evolution.

It was Darwin who first took a hard look at these patterns. Through his own youthful travels on the HMS Beagle and his voluminous correspondence with scientists and naturalists, he realized that evolution was necessary to explain not just the origins and forms of plants and animals but also their distributions across the globe. These distributions raised a lot of questions. Why did oceanic islands have such odd and unbalanced floras and faunas compared to continental assemblages? Why were nearly all of Australia's native mammals marsupials, while placental mammals dominated the rest of the world? And if species were created, why did the creator stock distant areas having similar terrain and climate, like the deserts of Africa and of the Americas, with species that were superficially similar in form but showed other, more fundamental differences?

Pondering these questions, others before Darwin laid the groundwork for his own intellectual synthesis—one he considered so important that it occupies two whole chapters in The Origin. These chapters are often considered the founding document of the field of biogeography—the study of the distribution of species on Earth. And their evolutionary explanation of the geography of life, largely correct when first proposed, has only been refined and supported by a legion of later studies. The biogeographic evidence for evolution is now so powerful that I have never seen a creationist book, article, or lecture that has tried to refute it. Creationists simply pretend that the evidence doesn't exist.

Ironically, the roots of biogeography lie deep in religion. The earliest "natural theologians" tried to show how the distribution of organisms could be reconciled with the account of Noah's Ark in the Bible. All living animals were understood as the descendants of the pairs that Noah took aboard, pairs that traveled to their present locations from the Ark's post-flood resting place (traditionally near Mount Ararat in eastern Turkey). But this explanation had obvious problems. How did kangaroos and giant earthworms make their way across the oceans to their present home in Australia? Wouldn't the pair of lions have quickly made a meal of the antelopes? And as naturalists continued to discover new species of plants and animals, even the staunchest believer realized that no boat could possibly hold them all, much less their food and water for a six-week voyage.

So another theory arose: that of multiple creations distributed across the Earth's surface. In the mid-i8oos, the renowned Swiss zoologist Louis Agassiz, then at Harvard, asserted that "not only were species immutable and static, but so were their distributions, with each remaining at or near their site of creation." But several developments also made this idea untenable, especially the increasing number of fossils disproving the claim that species were "immutable and static." Geologists such as Charles Lyell, Darwin's friend and mentor, began to find evidence that the Earth was not only very old, but in flux. On the Beagle voyage, Darwin himself discovered fossil seashells high in the Andes, proving that what is now mountain was once underwater. Lands could rise or sink, and the continents we see today might have been larger or smaller in the past. And there were those unanswered questions about the distribution of species. Why was the flora of southern Africa so similar to that of southern South America? Some biologists proposed that all the continents were once connected by giant land bridges (Darwin grumbled to Lyell that these bridges were conjured up "as easily as a cook does pancakes"), but there was no evidence that they had ever existed.

To deal with these difficulties, Darwin proposed his own theory. The distributions of species, he claimed, were explained not by creation, but by evolution. If plants and animals had ways of dispersing over large distances and could evolve into new species after they dispersed, then this—combined with some ancient shifts in the Earth, like periods of glacial expansion—could explain many peculiarities of biogeography that had puzzled his predecessors.

Darwin turned out to be right—but not completely. True, many facts about biogeography made sense if one assumed dispersal, evolution, and a changing Earth. But not every fact. The large flightless birds, like ostriches, rheas, and emus, occur in Africa, South America, and Australia, respectively. If they all had a common flightless ancestor, how could they have possibly dispersed so widely? And why do eastern China and eastern North America—widely separated areas—share plants, like tulip trees and skunk cabbage, that don't occur in the intervening lands?

We now have many of the answers that once eluded Darwin, thanks to two developments that he could not have imagined: continental drift and molecular taxonomy. Darwin appreciated that the Earth had changed over time, but he had no idea of how much change had actually taken place. Since the 1960s, scientists have known that the past geography of the world was very different from that of the present, as huge supercontinents have shifted about, joined, and separated into pieces.22

And, starting about forty years ago, we have accumulated information from DNA and protein sequences that tell us not only the evolutionary relationship between species, but also the approximate times when they diverged from common ancestors. Evolutionary theory predicts, and data support, the notion that as species diverge from their common ancestors, their DNA sequences change in roughly a straight-line fashion with time. We can use this "molecular clock," calibrated with fossil ancestors of living species, to estimate the divergence times of species that have poor fossil records.

Using the molecular clock, we can match the evolutionary relationships between species with the known movements of the continents, as well as the movements of glaciers and the formation of genuine land bridges such as the Isthmus of Panama. This tells us whether the origins of species are concurrent with the origin of new continents and habitats. These innovations have transformed biogeography into a grand detective story: using a variety of tools and seemingly unconnected facts, biologists can deduce why species live where they do. We know now, for instance, that the similarities between African and South American plants are not surprising, for their ancestors once inhabited a supercontinent—Gondwana—that split into several pieces (now Africa, South America, India, Madagascar, and Antarctica) beginning about 170 million years ago.

Every bit of biogeographic detective work turns out to support the fact of evolution. If species didn't evolve, their distributions on Earth, both living and fossil, wouldn't make sense. We'll look first at species that live on continents and then at those on islands, for these disparate areas provide different sorts of evidence.

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